Document [a; 0] behavior.

Author: ehuss

src/expressions/array-expr.md

@@ -16,14 +16,21 @@ brackets. This produces an array containing each of these values in the
 order they are written.
 
 Alternatively there can be exactly two expressions inside the brackets,
-separated by a semi-colon. The expression after the `;` must have type
+separated by a semicolon. The expression after the `;` must have type
 `usize` and be a [constant expression],
 such as a [literal](../tokens.md#literals) or a [constant
 item](../items/constant-items.md). `[a; b]` creates an array containing `b`
-copies of the value of `a`. If the expression after the semi-colon has a value
+copies of the value of `a`. If the expression `b` after the semicolon has a value
 greater than 1 then this requires that the type of `a` is
 [`Copy`](../special-types-and-traits.md#copy), or `a` must be a path to a
-constant item.
+constant item. When `b` evaluates to 0, the expression `a` is evaluated once
+unless it is a path to a constant item, in which case the constant is not
+instantiated.
+
+> **Note:** In the case where `b` is 0, and `a` is a non-const expression,
+> there is currently a bug in `rustc` where the value `a` is evaluated but not
+> dropped, thus causing a leak. See [issue
+> #74836](https://github.com/rust-lang/rust/issues/74836).
 
 ```rust
 [1, 2, 3, 4];